Install system python3-crcmod on Travis.

Python3-setuptools seems old and installing crcmod (dependency from
tockloader) from pip fails because of this. Installing from the system
package manager should solve the problem.

.travis.yml

@@ -25,6 +25,7 @@ addons:
       - "python3"
       - "python3-pip"
       - "python3-setuptools"
+      - "python3-crcmod"
 
 cache:
   - rust